1. Technical Field
The present invention relates to an image projection apparatus.
2. Background Art
Image projection apparatuses known as projectors have been widely used with the advancement of higher resolution of liquid crystal panels, enhancement of brightness of light source lamps due to higher efficiency, and lower price. Further, light-weight and compact image projection apparatuses using digital micro-mirror devices (DMD) as an image generation element have been widely used in offices, schools, and homes. For example, front type projectors having enhanced portability have been used for small meetings of several persons.
Further, projectors are demanded to project a larger image while also demanded to be compact for a projection space required outside the projectors. Recently, short focus projectors having a projection distance of 1 m to 2 m and a projection size of 60 inches to 80 inches have been developed. When projectors having a longer projection distance are placed on a table in a meeting room, the projectors need to be placed at position on the table far from a screen. By contrast, the short focus projectors can be placed near the screen on a table, with which a rear space of projectors on the table can be used freely.
The image projection apparatus includes a light source such as a lamp. Temperature in the image projection apparatus increases when the light source is ON. Therefore, the image projection apparatus includes a cooling unit such as an air-intake port and an air-discharge port to prevent temperature increase, and fans such as an air-intake fan and an air-discharge fan are disposed near each of the air-intake port and air-discharge port to generate forced air draft.
Further, the image projection apparatus having the air-intake port and air-discharge port includes an optical radiation system and an optical projection system, wherein the optical radiation system radiates light emitted from the light source to an image generation element, and the optical projection system projects an image generated by the image generation element to a projection face. As to this image projection apparatus, dust in air may intrude inside the image projection apparatus through the air-intake port and air-discharge port, and then may stay in a light path in the optical radiation system and the optical projection system, with which images having error such as poor quality images may be projected.
In view of this dust intrusion into the apparatus, an air filter can be provided before the air-intake fan to reduce an amount of dust intrusion, and optical parts can be sealed so that dust does not intrude in the optical radiation system and the optical projection system.
However, when the air filter is disposed before the air-intake fan, an air-intake area decreases, with which wind amount decreases and cooling performance in the apparatus decreases. In this case, rotation speed of the fan can be increased to maintain the cooling performance, but the increased rotation speed increases noise. Further, when dust adheres on the air filter, the cooling performance decreases, in which the apparatus is stopped and a user needs to clean and replace the air filter periodically, which is not convenient.
When the optical parts are sealed to prevent dust intrusion in the optical radiation system and the optical projection system, dust does not adhere on a light path in the optical radiation system and the optical projection system, and a normal projection image can be generated. However, some parts cannot be sealed. For example, a color wheel and a light tunnel disposed between the light source and the optical radiation system cannot be sealed because they are required to be cooled by air. Therefore, if dust adheres inside the light tunnel, luminance of projection image decreases.